1.1 Energy stores and systems Flashcards
System
An object or group of objects.
When a system is in equilibrium, nothing changes so no energy is transferred.
When there is a change in a system, things happen so energy is transferred.
Thermodynamic systems
An open system allows the exchange of energy and matter to or from its surroundings.
A closed system can exchange energy but not matter to or from its surroundings.
An isolated system does not allow the transfer of matter or energy to or from its surroundings.
Conservation of energy
Energy cannot be created or destroyed, it can only be transferred from one store to another. This means that for a closed system, the total amount of energy is constant.
8 energy stores
Kinetic, gravitational, elastic, magnetic, electrostatic, chemical, nuclear, thermal.
Energy transfer pathways
Energy is transferred between stores via transfer pathways.
Mechanical - when a force acts on an object.
Electrical - a charge moving through a potential difference.
Heating - energy is transferred from a hotter object to a colder one (conduction)
Radiation - energy transferred by electromagnetic waves.
Kinetic energy
The amount of energy an object has as a result of its mass and speed.
Ek = ½mv^2
Ek = kinetic energy in joules (J)
m = mass of the object in kilograms (kg)
v = speed of the object in metres per second (m/s)
Gravitational potential energy
The energy an object has due to its height in a gravitational field.
Eg = mgh
Eg = gravitational potential energy, in joules (J)
m = mass, in kilograms (kg)
g = gravitational field strength in newtons per kilogram (N/kg)
h = height in metres (m)
Elastic potential energy
The energy stored in an elastic object when work is done on the object.
Ee = ½ke^2
Ee = elastic potential energy in joules (J)
k = spring constant in newtons per metre (N/m)
e = extension in metres (m)
Thermal energy
Energy in the thermal store of an object is responsible for its temperature.
ΔE = mcΔt
ΔE = change in energy, in joules (J)
m = mass, in kilograms (kg)
c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg °C)
Δt = change in temperature, in degrees Celsius (°C)
Practical 1 (specific heat capacity)
IV - Energy supplied
DV - Temperature
CV - Mass, insulation, power
Procedure - Start by assembling the apparatus, placing the heater into the top of the block.
Measure the initial temperature of the aluminium block from the thermometer.
Turn on the power supply and start the stopwatch.
Whilst the power supply is on, the heater will heat up the block. Take several periodic measurements.
Switch off the power supply, stop the stopwatch and leave the apparatus for about a minute. The temperature will still rise before it cools.
Monitor the thermometer and record the final temperature reached for the block.
Power
The rate of this energy transfer, or the rate of work done
P = E/t or P = W/t
P = power in watts (W)
E = energy transferred in joules (J)
t = time in seconds (s)
W = work done in joules (J)
Wasted energy
When energy transfers occur that are not useful, these are described as energy being dissipated to the surroundings.
This is considered to be wasted energy.
Often these less useful energy transfers involve heating, light and sound.
Reducing unwanted energy transfers
Lubrication - Friction is a major cause of wasted energy in machines.
This wasted energy can be reduced if the amount of friction can be reduced.
This can be achieved by lubricating the parts that rub together.
Insulation - In many situations, the energy transferred by heating is wanted.
If this energy can be prevented from dissipating, then less energy will be needed to replace the wasted energy.
This can be achieved by surrounding the appliance with insulation.
Efficiency
The efficiency of a system is a measure of the amount of wasted energy in an energy transfer.
Efficiency = (useful energy output/total energy input) x 100
Practical 2 (investigating insulation)
IV - Insulating material
DV - Temperature
CV - Volume of water, initial temperature
Procedure - Set up the apparatus by placing a small beaker inside the larger beaker.
Fill the small beaker with boiling water from a kettle.
Place a piece of cardboard over the beakers as a lid. It should have a hole suitable for a thermometer and place the thermometer through this hole and into the water in the small beaker.
Record the temperature of the water in the small beaker and start the stopwatch.
Record the temperature of the water every 2 minutes for 20 minutes, or until the water reaches room temperature.
Repeat the experiment, each time changing the cardboard for another insulating material and also without any insulation at all.
